Precipitation reactions -- reactions in which two solutions are mixed and a solid product separates from the resulting liquid mixture -- as a mainstay of the AP reactions questions. These questions effectively ask students to learn a set of solubility rules. Aside from groups I and II metals (and ammonium ion), the metallic sulfides are insoluble. are discussed elsewhere.

"Rules" can be applied sequentially to determine solubilities. These rules are neither all inclusive nor completely accurate!

All:

0. sodium, potassium, and ammonium salts are soluble
   
1. nitrates, acetates, and perchlorates are soluble
   
2. silver, lead, and mercury(I) salts are insoluble
   
3. chlorides, bromides, and iodides are soluble
   
4. carbonates, chromates, hydroxides, oxides, phosphates, and sulfides are insoluble
   
5. sulfates are soluble (except CaSO₄, SrSO₄ and BaSO₄).

The reactions selected for testing are not obscure, especially when viewed from the perspective of a teacher offering a rich hands-on and demonstration experience for AP students. At the same time, there is a great deal of information to know!